A multidisciplinary
team, composed of stratigraphers, petrophysicists, reservoir engineers,
and geophysicists, studied a portion of Boonsville gas field in the Fort
Worth Basin of north-central Texas to determine how modern geophysical,
geological, and engineering techniques can be combined to understand the
mechanisms by which fluvio-deltaic depositional processes create reservoir
compartmentalization in a low- to moderate-accommodation basin. An extensive
database involving well logs, cores, production, and pressure data from
more than 200 wells, 26 mi
2
(67 km
2
) of 3-D seismic data, vertical seismic profiles (VSPs), and checkshots
was assembled to support this investigation. We found the most important
geologic influence on stratigraphy and reservoir compartmentalization
in this basin to be the existence of numerous karst collapse chimneys
over the 26-mi
2
(67 km
2
) area covered by the 3-D seismic grid. These near-vertical karst collapses
originated in, or near, the deep Ordovician-age Ellenburger carbonate
section and created vertical chimneys extending as high as 2500 ft (610
m) above their point of origin, causing significant disruptions in the
overlying clastic strata.

These karst disruptions
tend to be circular in map view, having diameters ranging from approximately
500 ft (150 m) to as much as 3000 ft (915 m) in some cases. Within our
study area, these karst features were spaced 2000 ft (610 m) to 6000 ft
(1830 m) apart, on average. The tallest karst collapse zones reached into
the Middle Pennsylvanian Strawn section, which is some 2500 ft (760 m)
above the Ellenburger carbonate where the karst generation began.

We used 3-D seismic
imaging to show how these karst features affected the strata above the
Ellenburger and how they have created a well-documented reservoir compartment
in the Upper Caddo, an upper Atoka valley-fill sandstone that typically
occurs 2000 ft (610 m) above the Ellenburger. By correlating these 3-D
seismic images with outcrops of Ellenburger karst collapses, we document
that the physical dimensions (height, diameter, cross-sectional area)
of the seismic disruptions observed in the 3-D data equate to the karst
dimensions seen in outcrops. We also document that this Ellenburger carbonate
dissolution phenomenon extends over at least 500 mi (800 km), and by inference
we suggest karst models like we describe here may occur in any basin that
has a deep, relatively thick section of Paleozoic carbonates that underlie
major unconformities.

1
Geophysics, Vol. 61, No. 5, pp. 13361350, 11 figs., 1 table.

2
Bureau of Economic Geology, The University of Texas at Austin, Box X,
University Station, Austin, Texas 78713; e-mail: bob.hardage@beg.utexas.edu.